1. Field of the Invention
The present invention relates to a rotational fluctuation malfunction detection device for an internal combustion engine and to a rotational fluctuation malfunction detection method for the internal combustion engine.
2. Description of the Related Art
In general, combustion strokes (expansion strokes) occur periodically when an internal engine is running. For example, in a 4 cylinder 4 cycle internal combustion engine the combustion stroke occurs at every 180° of a crank angle. Accordingly, the rotational speed of an output shaft (crankshaft) of the internal combustion engine generally fluctuates uniformly in a periodic manner in which the rotational speed increases during the combustion stroke and decreases outside of the combustion stroke. The fluctuation in rotational speed that occurs periodically with the combustion strokes is the normal behavior of the engine. Therefore, such fluctuations will be referred to as “normal fluctuations.” The rotational speed of the output shaft of the internal combustion engine may simply be referred to as “output shaft rotational speed.”
For various reasons, non-uniform fluctuations in the output shaft rotational speed sometimes occur that deviate excessively from the normal fluctuations. For example, in a multiple cylinder internal combustion engine, a fuel injection valve for one of the cylinders may characteristically inject a smaller amount of fuel than specified while the fuel injection valves for the other cylinders inject the specified amount of fuel. Accordingly, the deviant cylinder is called “lean imbalanced cylinder,” and the other cylinders are called “normal cylinders.” And, the above state is called “lean imbalanced state.”
In the lean imbalanced state, the combustion energy in the lean imbalanced cylinder is smaller than that of a normal cylinder. In other words, the peak speed of the combustion stroke of the lean imbalanced cylinder may be below the peak speed of the combustion stroke in a normal cylinder. As a result, non-uniform rotational speed fluctuation may occur. If the lean imbalance is large (when the fuel injection amount of the lean imbalanced cylinder deviates significantly from the specific amount), the rotational speed fluctuation may become excessively non-uniform.
In addition, if a fuel injection valve in a particular cylinder injects more than the specified amount of fuel, non-uniform fluctuations in the output shaft rotational speed may also be induced that deviate excessively from the normal fluctuations. The above-described specific cylinder may be referred to as a “rich imbalanced cylinder,” and the above-described state is called “rich imbalanced state.” However, the extent of non-uniformity in the rich imbalanced state is generally not as excessive as that which occurs in the lean imbalanced state. The reason is that the difference in combustion energy between the imbalanced cylinder and the normal cylinder is less significant in the rich imbalanced state than in the lean imbalanced state. Conclusively, the excessive lean imbalanced state easily causes the excessively non-uniform rotational speed fluctuation.
The rotational speed fluctuation becomes excessively non-uniform if a malfunction occurs in a torsional damper interposed between the output shaft of the internal combustion engine and the input shaft of the transmission (automatic transmission) to prevent the pulsation of the torque transmitted to the transmission from the internal combustion engine. In addition, the rotational speed fluctuation may become excessively non-uniform if misfire occurs within one of the cylinders.
Accordingly, if the rotational speed fluctuation becomes excessively non-uniform for the reasons described above, countermeasures need to be taken. For this reason, it is necessary to detect a “malfunction” in the vehicle that causes the excessive non-uniform rotational speed fluctuation. For example, Japanese Patent Application Publication No. 2004-346807 (JP-A-2004-346807) describes a device that outputs signals indicating the determination of whether the degree in non-uniformity of rotational speed fluctuation is abnormal based on the value of the parameter (referred to as “fluctuation parameter” hereinafter) that indicates the degree of non-uniformity of the rotational speed fluctuation.
When the vehicle is in motion (and when the transmission of the vehicle is set into a driving range other than neutral), rotational speed fluctuation tends to occur due to disturbances from the driving wheels of the vehicle. This tendency increases when the vehicle travels over a rough road. Hereinafter, the above-mentioned state, in which the rotational speed fluctuates due to disturbances from the driving wheel while the internal combustion engine is running, is called “disturbance fluctuation state.” When the vehicle is stopped (it does not matter whether the transmission is set into the driving range or neutral), the driving wheel does not move. In this case, the rotational speed fluctuation does not occur due to disturbances from the driving wheel of the vehicle. Hereinafter, the state, in which the rotational speed does not fluctuate due to disturbances from the driving wheel while the internal combustion engine is running, is called “non-disturbance fluctuation state.”
In the disturbance fluctuation state, the rotational speed fluctuation becomes non-uniform due to disturbances from the driving wheel. Therefore, if the fluctuation parameter is obtained in the disturbance fluctuation state, the “malfunction” (malfunction of rotational speed fluctuation) that indicates the rotational speed fluctuation be excessively non-uniform is possibly detected even when the malfunction does not really occur in the vehicle system. For this reason, it is preferred that the fluctuation parameter is obtained in the non-disturbance fluctuation state (such as, preferably, idling state of the vehicle) in which the rotational speed fluctuation is not affected by the disturbances from the driving wheel.
However, recently developed vehicles are able to stop the operation of the internal combustion engine under prescribed conditions while the vehicle is stopped. Such an idle stop function can be equipped not only in the vehicles that are driven by an internal combustion engine alone but also in the vehicles that are driven by an internal combustion engine and an electric motor together (so-called hybrid vehicles).
Vehicles equipped with the idle stop function rarely reach the state in which the internal combustion engine is continuously operated while the vehicle is stopped (that is, non-disturbance fluctuation state). In other words, when the fluctuation parameter is obtained in the non-disturbance fluctuation state, as described above, the opportunity to determine whether an malfunction of rotational speed fluctuation is occurring is problematically rare.